1. Technical Field
The present invention relates to an anode for use in PEM fuel cells, and to fuel cells comprising said anode, having improved tolerance to voltage reversal.
2. Description of the Related Art
Fuel cell systems are currently being developed for use as power supplies in numerous applications, such as automobiles and stationary power plants. Such systems offer promise of delivering power economically and with environmental and other benefits. To be commercially viable, however, fuel cell systems should exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside their preferred operating ranges.
Fuel cells convert reactants, namely, fuel and oxidant, to generate electric power and reaction products. Polymer electrolyte membrane fuel cells (“PEM fuel cell”) employ a membrane electrode assembly (“MEA”), which comprises a solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Separator plates, or flow field plates for directing the reactants across one surface of each electrode substrate, are disposed on each side of the MEA.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. (End plate assemblies are placed at each end of the stack to hold the stack together and to compress the stack components together. Compressive force effects sealing and provides adequate electrical contact between various stack components.) Fuel cell stacks can then be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
In practice, fuel cells need to be robust to varying operating conditions, especially in applications that impose numerous on-off cycles and/or require dynamic, load-following power output, such as automotive applications. For example, fuel cell anode catalysts are also preferably tolerant to cell voltage reversals; carbon-supported catalysts are also preferably resistant to corrosion during start up and shutdown procedures.
PEM fuel cells typically employ noble metal catalysts, and it is well known that such catalysts, particularly platinum, are very sensitive to carbon monoxide poisoning. This is a particular concern for the anode catalyst of fuel cells operating on reformate; but it also a concern for fuel cells operating on hydrogen, as CO is sometimes present in the hydrogen supply as a fuel contaminant and/or as a result of membrane cross-over from the oxidant supply in applications where air is employed. As described by, e.g., Niedrach et al. in Electrochemical Technology, Vol. 5, 1967, p. 318, the use of a bimetallic anode catalyst comprising platinum/ruthenium, rather than monometallic platinum, shows a reduction in the poisoning effect of the CO at typical PEM fuel cell operating temperatures. Hence, Pt—Ru catalysts are typically employed as PEM fuel cell anode catalysts.
The anode layer of PEM fuel cells typically includes catalyst and binder, often a dispersion of polytetrafluoroethylene (PTFE) or other hydrophobic polymer, such as described in U.S. Pat. No. 5,395,705, and may also include a filler (e.g., carbon). Anode layers are also described that comprise catalyst and an ionomer (e.g., U.S. Pat. No. 5,998,057) and a mixture of catalyst, ionomer and binder (e.g., U.S. Pat. No. 5,242,765). The presence of ionomer in the catalyst layer effectively increases the electrochemically active surface area of the catalyst, which requires an ionically conductive pathway to the cathode catalyst to generate electric current.
Voltage reversal occurs when a fuel cell in a series stack cannot generate sufficient current to keep up with the rest of the cells in the series stack. Several conditions can lead to voltage reversal in a PEM fuel cell, for example, including insufficient oxidant, insufficient fuel, insufficient water, low or high cell temperatures, and certain problems with cell components or construction. Reversal generally occurs when one or more cells experience a more extreme level of one of these conditions compared to other cells in the stack. While each of these conditions can result in negative fuel cell voltages, the mechanisms and consequences of such a reversal may differ depending on which condition caused the reversal. Groups of cells within a stack can also undergo voltage reversal and even entire stacks can be driven into voltage reversal by other stacks in an array. Aside from the loss of power associated with one or more cells going into voltage reversal, this situation poses reliability concerns. Undesirable electrochemical reactions may occur, which may detrimentally affect fuel cell components. Component degradation reduces the reliability and performance of the affected fuel cell, and in turn, its associated stack and array.
One approach for improving cell reversal tolerance is to employ a catalyst that is more resistant to oxidative corrosion, by using higher catalyst loading or coverage on the anode catalyst support or a more oxidation resistant anode catalyst support, such as a more graphitic carbon or Ti4O7, as described in U.S. 2004/0157110. Conversely, U.S. 2006/0019147 discloses a catalyst layer where Pt and/or Pt alloy powder and carbon powder exist independently from each other.
As described in U.S. Pat. No. 6,936,370, fuel cells can also be made more tolerant to cell reversal by promoting water electrolysis over anode component oxidation at the anode. This can be accomplished by incorporating an additional catalyst composition at the anode to promote the water electrolysis reaction. During reversal, water present in the anode catalyst layer can be electrolyzed and oxidation (corrosion) of anode components, including carbon catalyst supports, if present, can occur. It is preferred to have water electrolysis occur rather than component oxidation. Thus, by incorporating a catalyst composition that promotes the electrolysis of water, more of the current forced through the fuel cell during voltage reversal can be consumed in the electrolysis of water than the oxidation of anode components. Among the catalyst compositions disclosed were Pt—Ru alloys, RuO2 and other metal oxide mixtures and/or solid solutions including Ru.
U.S. 2004/0013935 also describes an approach to improving cell voltage reversal tolerance by using anodes employing both a higher catalyst loading (at least 60 wt % catalyst) on an optional corrosion-resistant support, and incorporating certain unsupported catalyst compositions to promote the water electrolysis reaction. Disclosed preferred compositions include oxides characterized by the chemical formulae RuOx and IrOx, where x is greater than 1 and particularly about 2, and wherein the atomic ratio of Ru to Ir is greater than about 70:30, and particularly about 90:10.
However, Ru has been shown to be unstable under certain fuel cell operating conditions. For example, Piela et al. (J. Electrochem. Soc., 151 (12), A2053-A2059 (2004)), describe Ru crossover from Pt—Ru black catalyst and redeposition at the Pt cathode catalyst in direct methanol fuel cells (DMFC) and hydrogen/air fuel cells under abnormal conditions, such as cell reversal resulting in very high anode potentials (and under normal DMFC operating conditions). Piela et al. theorized that the Pt—Ru alloy should likely remain stable under DMFC operating conditions, and that the source of the Ru contamination was neutral hydrous RuO2. Taniguchi et al. (J. Power Sources, 130, 42-49 (2004)) observed Ru dissolution from a carbon supported Pt—Ru anode catalyst as a result of high anode potentials experienced by the fuel cell under cell reversal conditions. Unlike Piela et al., Taniguchi et al. did not detect Ru in the membrane or cathode side of the fuel cells tested.
In contrast, U.S. 2004/0214058 discloses that the increase in anode potential during fuel shortage causes the formation of some film on the surface of the catalyst that reduces its activity. A multilayered electrode structure is proposed in which a layer for preferentially promoting the electrolysis of water during fuel shortage is provided so as to prevent the occurrence of water electrolysis in the region for advancing the fuel cell reaction, as a means for suppressing the observed performance reduction. The embodiments disclosed in US 2004/0214058 employ a Pt—Ru catalyst in the reaction layer, and a Pt catalyst in the water decomposition layer.
It is desirable to have a fuel cell anode that is more robust to operating conditions that impose numerous on-off cycles and/or require dynamic, load-following power output; are tolerant to cell voltage reversals; and resistant to corrosion during start up and shutdown procedures. The present invention addresses this need and provides associated benefits.